1. Field of the Invention
This invention relates to a dye thin film including a novel compound useful, for example, in optical switching and a production method thereof, as well as an optical switch, an optical distributor, an optical modulator and an optical switch array formed by use of the thin film described above.
2. Description of the Related Art
It has been known that some dye derivatives, particularly, cyanine, porphyrin and squarylium dye derivatives form an aggregate. The aggregate means the body in which several tens to several hundreds of molecules are regularly arranged and bonded loosely and behave optically as if they were a single super molecule. Particularly, the one shown in FIG. 5 having an absorption band FIG. 5B which is shifted toward a longer wavelength side and sharpened as compared with an absorption band FIG. 5A of a single molecule is referred to as J-aggregate.
It has been reported that the J-aggregate emits fluorescence with small Stokes shift, has extremely large interaction with light of a wavelength near the absorption peak and shows extremely rapid recovery from absorption saturation as a third-order nonlinear optical effect (M. Furuki, L. S. Pu, F. Sasaki, S. Kobayashi and T. Tani, Appl. Phys. Lett., 72, 21 (1998) 2648). Further, it has been confirmed that the optical response time of a thin film of squarylium dye aggregates formed on a solid substrate is 300 fs (1 fs=10−15 second) or less (Page 1 of Nikkan Kogyo Shimbun, dated Jul. 13, 1998). Further, a thin film of aggregates of a squarylium dye derivative having an ultrafast response characteristic of less than 100 fs which can be driven at an energy as low as 80 fJ/μm2 has been attained (Page 5 of Nikkei Sangyo Shimbun, dated Jul. 8, 1999; M. Furuki, M Tian, Y, Sato, L. S. Pu, H. Kawashima, S. Tatsuura and O. Wada, Appl. Phys. Lett., 78, 18(2001) 2634). With the feature described above, it is considered that a thin film of aggregates of a squarylium derivative can be used as an optical switch in optical information communication of terabit (1012 bit/s) order.
The present inventors have confirmed that the squarylium dye derivative forms aggregates on a solid substrate and they exhibit an ultrafast optical response characteristic of femtosecond order, for which patent applications were already filed (Japanese Published Unexamined Patent Application No. Hei 11-282034 and Japanese Published Unexamined Patent Application No. 2000-111967). However, since the ultrafast optical response of such a dye aggregate film can be obtained only for light of a wavelength near the absorption peak of the aggregates (780 nm), it is difficult at present to apply the film to optical switches that operate at a wavelength exceeding 1 μm. Since the wavelength of the signal light in actual optical fiber communication networks is 1.3 μm or 1.55 μm, it is essential for the optical switch applicable to a communication system to use a dye having an intense absorption at a wavelength exceeding 1 μm. On the other hand, there have so far been several kinds of near-infrared-absorbing dyes having maximum absorption wavelengths of 1 μm or more (J. Fabian, H. Nakazumi, M. Matsuoka., Chem. Rev, 92 (1992) 1197).
However, the long-wavelength-absorbing dyes have a feature that the dye molecule includes a large π-conjugated system, and have the following problems:    1) The molar absorption coefficient of the dye is small.    2) The thermal stability is poor since the π-conjugated system is excessively long.    3) The solubility in organic solvents is poor, making it difficult to form a film.
Accordingly, the existent long-wavelength-absorbing dyes are difficult to apply to optical switches. In order to solve this subject, it is necessary to make improvements such as an increase of the solubility of the long-wavelength-absorbing dye, improvement of the thermal stability and sublimation property of the dye and control for the aggregation of the dye molecules in the film.
For the reasons described above, there has been a demand for a dye thin film that contains a near-infrared-absorbing dye having a maximum absorption wavelength of over 1 μm and having high thermal stability and good solubility, exhibits intense absorption at a wavelength exceeding 1 μm and has an ultrafast optical response characteristic.